Prior to installation into computer disk drives, magnetic disks or a set of disks combined into so called disk pack has to be tested with respect to their basic characteristics such as a signal-to-noise ratio, track-to-track error, sticking of the head to the disk surface at starting of disk rotation, friction between the magnetic head and the surface of the magnetic medium, etc. The tests which are aimed specifically at testing and measuring aforementioned friction and rotation starting characteristics are known as start-stop tests.
In order to better understand the principle of the present invention, it would be useful to explain in a simplified form the construction and principle of operation of a conventional hard disk drive. A hard disk drive consists of a housing which contains a spindle motor which rotatingly supports a disk or a disk pack. The housing also supports a head or a head stack which is secured on a pivotally supported arm capable of performing swinging motions so that the magnetic head or heads can scan the working surfaces of the disk or disks for writing or reading the information recorded on the disk/disks. The arm is driven and positioned by means of a so called voice coil motor which in fact is an electric linear motor. The disks are rotated with a high speed. Each magnetic disk has to concentric zones. i.e., a head loading zone and a working zone. When the disk drive is switched into operation, the disk begin to rotate, and the pivot arm is positioned over the head loading zone. The latter has a plurality of microscopic projections for reducing the friction and sticking force between the magnetic head and the disk surface. The rotation speed of the disk increases to the level at which an air cushion is formed between the head and the disk sufficient to support the head out of physical contact with the disk surface, the pivot arm with the magnetic head is shifted further to the working zone of the disk where appropriate read/write operations are performed. Upon completion of the working cycle of the disk drive, the pivot arm returns the magnetic head to the parking position over the loading zone.
The information is recorded on the disks in the form of magnetic signals arranged on concentric tracks. In order to ensure recording of large volume of information, the magnetic signals are recorded with a very high density, and tolerances or track-to-track distances are very tight, e.g., on the order of .+-.0.012 .mu.m. It is understood that under these conditions the magnetic head should be very accurately positioned over appropriate tracks, and this function is fulfilled by means of servo signals prewritten on the disks for forming aforementioned tracks. This is so-called disk-formatting operations with which all computer users are familiar when working with non-formatted floppy disks.
Multiple start-stop operations of the disk drive cause deterioration of the magnetic-medium surfaces of the disks as well as the working surfaces of the magnetic heads. In the disk drives the disks rotate in a confined space limited by strict requirements to the overall dimensions of the disk drive. Therefore the products of wear such as debris from the deterioration of the disks and heads are accumulated inside the disk drive housing and contaminate the working surfaces. Eventually these particles penetrate the read/write gap and may further operation of the disk drive impossible.
Therefore, it is extremely important to test start-stop characteristics of the magnetic heads and magnetic disks inside the disk drive housings.
Start-stop testers for testing friction characteristics and sticking properties in head-disk pair are known in the art. For example, Center for Tribology, Inc, Mountain View, Calif. has developed a start-stop (Model. HDI Reliability Test System [RTS]). This tester has base plate which supports a spindle-motor unit for rotation of a hard disk to be tested and an additional electric motor for swinging motions of an arm which supports a magnetic head. A strain gauge is attached to the point of connection between the head and head-supporting structure for measuring the friction and sticking forces generated between the head and the disk during testing. The tester is equipped with a measurement system for measuring aforementioned forces, number of working cycles, etc.
A main disadvantages of this tester is that the disk is tested in an environment different from that inside the disk drive housing an that the products of wear and deterioration are removed from the zone of contact between the head and the disk and therefore do not affect their performance characteristics. The known start-stop testers utilize various systems for accurate positioning of the magnetic head with respect to tracks of the hard disk. For example, it may be an external (i.e., located outside the disk drive) positioning system with the use of a stepper motor which has low positioning accuracy. For example, even with the use of a special gear mechanism, the accuracy of the stepper-motor positioning system cannot be better than 1 .mu.m. Other testers utilize optical encoders such as a standard device (Model LIP401R), produced, e.g., by Heidenhain Corporation, Schaunburg, Ill., USA. Although the optical encoders may provide high positioning accuracy, they are heavy and large in size.
The start-stop tests are carried out till the crash conditions, i.e., to the conditions at which further read/write operation becomes impossible. In order to monitor the head and the magnetic medium deterioration, the read/write operation should be performed after each start-stop cycle.
Another disadvantage of conventional start-stop head/disk testers is that they are capable of measuring friction and sticking forces only for a single magnetic head. This is because the friction and sticking forces are measured by means of a strain gage located at the point of connection of the head to the pivot arm. In other words, these testers are not applicable for testing friction and sticking characteristics of the head stacks as a unity.
Furthermore, the known testers require that separate measurement and positioning systems be used for measuring head/disk characteristics and or accurate positioning of the head with the respect to the disk tracks.
On the other hand, in a real disk drive the disk, which rotates with a very high speed of about 4000 to 10000 rpm or more, generates aerodynamic conditions which cannot be neglected and which exert an influence on the formation of the air gap, resistance to disk rotation, etc. An ideal condition would be if the start-stop test is conducted inside the disk drive. This condition, however, cannot be implemented in view of the large dimensions and heavy weight of the external positioning mechanisms. In case the actual head positioning mechanism of the disk drive is used for this purpose, it would be impossible to continue the test, since deterioration of measurement conditions anticipate the actual physical deterioration of the working head and disk surfaces. In other words, when the measurement conditions are deteriorated because of contamination of the track surfaces and of the head gap to the extent that further measurement is impossible, it also becomes impossible to obtain any data which occurs at he moment of physical crash of the aforementioned components of the tester.